1. Exploring Engineering
Chapter 16
Green Engineering

2. Green Engineering Renewable sources of energy
The sun delivers a huge amount of energy and power to earth, ~2 × 1017 kW.
How do we utilize renewable sources of energy?
Some of this direct sunlight can power solar heating and photovoltaic power.
About 0.002%of the sunlight is converted to wind energy every day
This wind can power windmills for direct electricity production and, by the weather cycle, water for hydroelectric power.
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3. Green Resources Hardly Touched
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4. Green Resources Power Source of Power Capital Costs
Photovoltaics
Direct Sun
Relatively expensive, & potential for small applications ($4750/kW)
Solar thermal electric
power plants
May use the Sun directly to produce steam; expensive per unit of power ($3150/kW)
Solar Thermal Heat
Direct household use; costs depend on specifics
Windmills
Wind (itself dependent on the Sun)
Large installations are economical; limited sites available ($1,200/kW)
Hydropower
Hydroelectric river dams, tidal power generators
Many large facilities in the world; acceptable sites declining ($1500/kW)
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5. Photovoltaic Effect Explained by Einstein (Nobel Prize for this!)
Color
Frequency
Wavelength
violet
668–789 THz
380–450 nm
blue
631–668 THz
450–475 nm
cyan
606–630 THz
476–495 nm
green
526–606 THz
495–570 nm
yellow
508–526 THz
570–590 nm
orange
484–508 THz
590–620 nm
red
400–484 THz
620–750 nm
Explained by Einstein (Nobel Prize for this!)
Take a hot wire and shine a light on it … an electric current is observed, if and only if, the color of the light is blue enough
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6. Photoelectric Effect
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7. Frequency, Wavelength & Energy
Speed of light connects wavelength  & frequency  by  = c = 3.00 × 108 m/s
Note units of frequency  = cycles/s or Hz (Hertz)
Planck’s relationship gives the energy of a photon of light, E = h in which Planck’s constant × eVs in which 1 eV =1.60 × joules.
1 electron volt = energy of one electron in a voltage gradient of 1 volt
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8. Frequency, Wavelength & Energy
Lightwave
Why use eV for energy? It’s because neither  nor  are convenient and we eventually want electrical volts for our devices.
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9. Comparison of Units Exploring Engineering Frequency Hz, 
Wavelength in m
Energy in J
Energy in eV
“Red” photon
4.3 × 1014
0.70
2.9 × 10−19
1.8
“Blue” photon
7.5 × 1014
0.40
4.1 × 10−19
2.6
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10. Photoelectric and Photovoltaic Effects
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11. Photovoltaic Effect
Solids like silicon exhibit the photovoltaic effect. Unlike the photoelectric effect there are trillions of energy levels so that an electron can jump from a “valence” band to a “conduction” band provided they have sufficient energy to leap the band gap.
Material
Symbol
Bandgap
(eV)
Crystalline silicon
c-Si
1.11
Amorphous silicon
a-Si
1.7
Germanium Ge
0.67
Indium
Antimonide
InSb
0.17
Diamond C
5.5
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12. Photons with energies > bandgap will cause electrons to flow
The Sun’s Spectrum
Photons with energies > bandgap will cause electrons to flow
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13. Limits To Efficiency
The lower the bandgap, the more solar spectrum can be used and the more efficient the photovoltaic device
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14. How Many Devices in Series?
If a device has only 1 volt/cell and you need to deliver 100 V, you need 100 cells in series. If a cell delivers bust 1 A and you need 100 A, you need 100 devices in parallel.
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15. Solar Heat
Conceptually simple and can best be used for small installations
Modest solar storage possible
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16. Wind Power Competitive and a clear winner in green energy
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17. Wind Power Basics
The maximum of the wind’s KE is ½ v2 per volume of air passing through the blades ( is the air density)
If the effective cross section of the windmill is A, the wind flow rate is Q = Av
Max power is ½ v3A
Down wind air flow reduces this to ½ Cpv3A with Cp known as the Betz coefficient and is limited to  0.593
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18. Wind Example
A modern commercial windmill has a diameter of 100. meters and operates in winds of just 5.00 m/s. If the Betz power coefficient is 0.50, what is the power produced by this windmill? Assume that the air density is 1.05 kg/m3. What wind speed is needed to achieve the windmill’s design power output of 2.5 MW?
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19. Wind Example
Pideal = ½ρν3A = ½ × 1.05 × × 7.85 × 103 [kg/m3] [m/s]3 [m2] = 5.15 × 105 [Nm/s] = 5.15 × 105 W = MW.
Pactual = 0.50 × MW = 0.26 MW
For 2.5 MW, wind speed = (2.50/0.26)0.333 × 5.0 = 10.6 m/s = 11 m/s
Doubling the wind speed multiplies the power produced by a factor of 8 (!) and thus dictates the location of wind power sites.
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20. Hydropower Indirect from the sun through evaporation and rain.
Most hydropower needs to invest in large scale civil works
Can be used for load balancing as well as power production
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21. Dams Can Be Enormous – Several Miles Long!
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22. Dams Can Be Beautiful - Boulder Dam
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23. Hydropower Power produced is straight forward.
For 100% efficient turbine
Most turbines are ~ 90% efficient
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24. Pumped Storage
The motors and generators in some hydropower systems are reversible. That is, the water turbine can act as a pump and the generator as an electric motor to drive the pump. A power management scheme assumes that you have excess power some of the time and a deficiency at other times. Suppose you have a small dam and a 10.0 MW generator. During the demand period of 6.00 hours, you can produce 10.0 MW, which essentially empties all the water from the impoundment behind the dam. During the night, 12.0 MW of excess power is available from the power grid. How many hours does it take to refill the dam if the pumping and the generator steps are both 100% efficient?
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25. Hydropower allows for energy storage – a big advantage.
Pumped Storage
Since the water from the dam during discharge should equal the water pumped back during recharge, then the recharge time should be where t1 = 6.00 hours. We can relate the flow rates by the dam power equation and hence Q1/Q2 = t2/t1 and then t2 = t1 × = 6.00 × = 5.30 hours.
Hydropower allows for energy storage – a big advantage.
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26. Green Energy Summary Solar energy can supply electricity (and heat)
Photovoltaic power tracks sunlight and backup is difficult
Heat is simple form of green energy
Wind power is a coming green source but tracks the wind making it impossible to rely on power leveling.
Hydropower produces electricity and can be used to load level. New sites are becoming rare.
Exploring Engineering